Determinations of the imaginary refractive index in the ultraviolet and visible spectral regions as determined from diffuse reflectance measurements are presented. Materials examined are carbon black and oxides of iron, lead, mercury, copper, manganese, and vanadium along with lead iodide and iron sulfide.

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Atmospheric particulate matter was examined to estimate the significance of free carbon as an absorber of near-ultraviolet, visible, and near-infrared radiation. Bulk and size-fractionated samples have been disassembled into acetone-soluble, water-soluble, and insoluble fractions. The absorption coefficients for these fractions, and for the insoluble material after removal of the free carbon by burning, have been measured. The results show that in the visible and near infrared, free carbon, although not a major component by mass, is by far the dominant absorbing material. These measurements in relation to otherresearch suggest that geographic variations in and anthropogenic contributions to the free-carbon content cause much of the variation in the absorption coefficient of atmospheric particulate samples.

It is convenient to measure the optical attenuation A of the combination of a layer of atmospheric particulate matter and the quartz fiber filter on which it has been collected. The problem of relating A to the absorption and scattering coefficients k and s of the particulate matter itself is treated as a problem in diffuse reflectance spectroscopy using the KubelkaMunk theory. The results show that although, in general, A is a nonlinear function strongly dependent on both s and k, for a limited range of s and sample thickness d, A can be a practically linear function of k. Fortunately, this range includes that common to atmospheric particulate samples. Furthermore, it is shown that if the filter's reflectance is sufficiently high, A can be nearly independent of s. This is in agreement with experimental and, for the limiting case when the substrate filter reflectance is unity, theoretical results obtained by other researchers. Use of such measurements of A as a means of determining the black carbon mass loading C on a filter is also investigated. It is shown that when the black carbon mass fraction f(c) is high, as it is for samples collected in large urban areas, A is a predictable and practically linear function of C. However, when f(c) is low, as it is for many rural locations, then the slope of the function A(C) is strongly dependent on f(c), leading to possible overestimates of C. This problem can be alleviated by making the measurement of A at near-infrared wavelengths rather than in the visible spectrum.

Measurements of aerosol size distributions, chemical composition, and optical properties were carried out at two locations in Israel. Tel Aviv was chosen as an urban region, and Mitzpe Ramon as a desert region. The measurements indicate that the size spectra between 0.15 and 12 μm can be approximated by a power law type of distribution. The spectra in Tel Aviv show higher concentrations and greater slopes than those from the desert. Diurnal variations between day and night are observed in the size distribution of the summertime measurements in Tel Aviv. These are believed to be related to the increase in aerosol size due to absorption of water vapor during high relative humidity periods. The chemical composition in Tel Aviv was found to depend strongly on the meteorological conditions. During westerly flow, which is the prevailing wind direction in Tel Aviv, the aerosols contain large amounts of NaCl particles, which are not usually found in the desert. During Sharav conditions (dry conditions and easterly flow) the aerosols in Tel Aviv are, as is expected, very similar to those found in the desert. The optical measurements also suggest strong differences between Tel Aviv and the desert. Filter samples of atmospheric particulate matter from Tel Aviv always appeared darker than those from the desert site. Measurements of imaginary refractive index in the 0.3‐ to 1.7‐μm spectral region are presented. These measurements are consistently higher by a factor of 2–4 in Tel Aviv as compared to simultaneous samples from the desert site. Seasonal variations also have been observed, and their possible causes are discussed. The size distribution, composition, and imaginary refractive index observations are interpreted in terms of aerosol origins and prevailing local and synoptic scale meteorological conditions.

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